This invention relates to an arrangement for automatically filtering contaminants and foreign materials from electroscopic developer material used in electrostatic copiers.
In the art of xerography, a xerographic plate, which is formed of a conductive backing upon which is placed a photoconductive insulating material is charged uniformly in the surface of the plate and subsequently exposed to a light image of the original to be reproduced. The photoconductive coating is thereby caused to become conductive under the influence of the light image so as to selectively dissipate the electrostatic charge found thereon thus producing an electrostatic latent image. The latent image is made visible by developing it with any one of a variety of pigmented resins which have been specifically developed for this purpose. In the xerographic process, the pigmented resin material, or toner, is electrostatically attracted to the latent image on the photoconductive surface in proportion to the amount of charge found thereon. Areas of small concentration become areas of low toner density while areas of greater charge concentration become proportionally more dense. The fully developed image is then transferred from the plate surface to the final support material, as for example, paper, is fixed thereto to form a permanent record of the original copy.
A preponderance of the toner material is transferred from the photoconductive surface to the final support material during the transfer operation. However, it has been found that forces bonding some of the toner particles to the photoconductive surface are stronger than the transfer forces involved and, therefore, some particulate material remains on the photoconductive surface after the xerographic image is transferred. This residual toner, if not cleaned from the xerographic plate in some manner, will have a deleterious effect on subsequent images processed on the plate.
Plate cleaning in automatic xerographic machines in which the plate is continually reused in the xerographic process is accomplished by various devices such as fiber brushes, cleaning webs, wiper blades or the like. The toner material so removed may be collected and stored in the machine and then periodically removed and discarded. Alternatively, collected toner may be returned from the cleaning station of the machine to the development housing for reuse in the development process. This returning of toner may be done manually by first collecting the cleaner toner in a container at the cleaning station and later dumping the contents of this container into the developer sump.
A system for automatically recovering residual toner and returning it to the developer housing for reuse in the development zone is described in U.S. Pat. No. 3,752,576 and U.S. Pat. No. 3,678,896 in which an endless bead chain conveyor moves between the cleaning station and the development station of a xerographic system. As provided in the cleaning systems shown in the above-noted patents, toner cleaned from the xerographic plate at the cleaning station is moved from the cleaning station to the developer station by means of a bead chain conveyor and deposited into the sump of the development unit for reuse in the development process.
It has been found that toner returned to the development unit in such systems often contains contaminants and foreign matter such as fibers, brush fibers, metal chips, and pieces of foam which if not removed are automatically transported back to the developer housing by the above noted bead chain system. Such foreign particles often jam up the bead chain system thus necessitating cleaning or replacement of the unit in the field. If the particles pass through the bead chain conveyor system, they are returned to the developer housing and may have an adverse effect on the quality of the development system. As developer life increases, the percentage of foreign particles being constantly recirculated within the machine in the above-noted manner gradually increases and the deleterious effects thereof become more aggravated. The only removal of contaminants under the present system occurs when either the developer is changed or the toner return system is removed and cleaned or replaced.